It is well known in the art to produce nitrogen via liquefaction and distillation of air. In a number of industrial processes nitrogen is utilized for purposes such as inerting, annealing during metal production, inerting tin baths during flat glass production and the like. In these operations it is conventional to have a nitrogen plant to satisfy the nitrogen requirements for the overall process. Conventional technology used to separate nitrogen from the air produces a vent stream that is oxygen rich and is usually vented to the atmosphere.
It is also known that in order to improve productivity and thermal efficiency of heating and melting processes, oxygen enhanced combustion is a valuable tool. Oxygen has been proven to provide a variety of benefits in combustion processes namely, increased productivity and fuel efficiency, reduced flue gas volume and pollution emission, and the like. Oxygen has been used in many ways to enhance existing air/fuel-fired combustion processes which fall into five broad categories, namely: enriching combustion air (sometime known as pre-mix or enrichment); lancing oxygen into air/fuel flames (lancing); combining an oxy-fuel burner together with an air/fuel burner (air-oxy/fuel); adding to and/or replacing some of the existing air/fuel burners with oxy/fuel burners (boosting); and complete replacement of all air/fuel burners with oxy/fuel burners (oxy/fuel). Although oxy/fuel typically results in the greatest benefit it usually comes at the highest cost to the user.
One technique of enhanced combustion is the use of intermediate purity oxidizers with a conventional burner. U.S. Pat. Nos. 4,797,087, 5,217,363, 5,308,239, 5,611,683 and 5,454,712 all involve processes using air and oxygen in a burner to produce an intermediate overall enrichment level. None of these patents disclose the use of a single oxidizer with an intermediate oxygen composition.
UK Patent Application 2 067 668 A describes a process wherein an air separation plant feeds oxygen to a coal gasification unit to produce a medium BTU fuel gas and feeds nitrogen into the incoming combustion air of a gas turbine. The fuel gas and oxygen deficient air (which is used to reduce NOx compared to normal air) are then fed into a gas turbine.
U.S. Pat. No. 5,388,645 discloses a process where an oxygen-containing gas (e.g. air) is separated into oxygen-enriched and oxygen-depleted streams. The oxygen depleted stream is injected into methane-containing solid carbonaceous subterranean formations to produce a methane-containing gaseous mixture which is referred to as coalbed methane. The oxygen depleted stream reduces corrosion and minimizes the chances for explosion when compared to a similar process using air. The oxygen-enriched stream is reacted with an oxidizable material, which could be methane containing steam. Using both streams improves the economics of the recovery process.
U.S. Pat. No. 5,459,994 describes a process wherein an air separation plant (ASU) is coupled with a gas turbine so that the nitrogen-rich air stream (85 to 100% nitrogen) from the air separation unit is added to the incoming combustion air stream of the turbine to reduce NOx emissions by using an oxidant with a lower oxygen concentration than that found in air. The process also increases the power output and thermal efficiency of the turbine because of the additional volume and pressure head of the nitrogen-rich stream. The oxygen-rich stream (80 to 100% oxygen) is used in a coal-gasification process to produce a fuel gas for use in the turbine.
European Patent Publication 0 474 198 B1 describes a process for improving the waste burnout and reducing emissions in a waste incinerator. Waste is fed into the incinerator onto a slanted grate. The main combustion zone has primary combustion air plus pure oxygen injected into it to produce oxygen-enriched air which includes utilization of up to and including pure oxygen, which is fed through the grate to improve burnout. The second part of the grate, known as the burnout zone has an inert gas, such as nitrogen blended into the secondary air stream. The nitrogen and oxygen may come from an air separation plant. The actual content of the secondary air stream is adjusted from being oxygen-deficient to oxygen-rich, depending upon the contents of the exhaust stream coming from the primary combustion zone. Adjustment of the oxygen content of the secondary stream is used to control the combustion intensity.
U.S. Pat. No. 4,460,558 discloses utilization of high purity oxygen and fuel reacted to produce carbon black where the waste gas nitrogen stream is also used downstream of the process to recover heat and as a purge to a dryer.
U.S. Pat. No. 5,069,692 discloses and claims a process for an aircraft system where an oxygen rich stream from an air separation unit is used for a combustor and a nitrogen rich stream is used for fuel tank safety to pump the fuel.
U.S. Pat. No. 5,076,837 discloses and claims a process for using both streams from an air separation unit where the oxygen rich stream is used in a chemical process (e.g. gasification) and a nitrogen rich stream is preheated by the exhaust products from the chemical process and then the preheated nitrogen is used to preheat one of the incoming chemicals for the chemical process.
Japanese Patent Application H8-285219 describes a process wherein high purity oxygen produced by a pressure swing adsorption oxygen generator is supplied to a burner and the nitrogen rich exhaust from the pressure swing adsorption apparatus is blended with the exhaust gases to be used to preheat the incoming raw materials and also to dilute the concentration of pollutants in the exhaust products.
The process in U.S. Pat. No. 4,827,716 utilizes the nitrogen which is the primary product stream from an air separation unit carried on board an aircraft to inert the fuel tanks while the oxygen rich stream produced by the same unit is used as an oxidant to drive an emergency power unit on the aircraft.